1. Field of the Invention
The present invention generally relates to soft switched three-phase converters and, more particularly, to three-phase rectifiers and inverters having improvements made to the DC rail side of the converter for improving performance, reliability and power factor correction (PFC).
2. Description of the Prior Art
Conventional diode and thyristor bridge rectifiers create strong harmonic currents which can pollute public utility networks. In an effort to protect utility quality, legislation has been proposed limiting rectifier harmonic output current. As such, companies that manufacture power electronics equipment are constantly looking for new power factor correction (PFC) techniques, and ways to integrate PFC into their products. FIGS. 1 and 2 are examples of prior art converters which offer PFC. FIG. 1 is a three-phase boost rectifier ideal for high power applications which offers unity power factor with continuous input currents. Here, three a.c. phases, V.sub.a, V.sub.b, and V.sub.c, are passed through a bridge switching network and over a smoothing capacitor C.sub.o to supply a d.c. load. On the opposite end of the spectrum, FIG. 2 shows a prior art three-phase voltage source inverter. Here, a d.c. voltage source V.sub.in is transformed by a bridge switching network into three-phase a.c. currents, i.sub.a, i.sub.b, and i.sub.c. This type of inverter is widely used in motor drives and Uninterrupted Power Supply (UPS) systems.
For both the rectifier in FIG. 1 and the inverter in FIG. 2, if no soft-switching technique is applied, the six bridge anti-parallel diodes will cause a severe reverse recovery problem due to a high DC rail voltage. For high power applications minority carrier switching devices, such as BJTs, IGBTs, GTOs are often used, which have severe turn-off current tail problem which further exacerbate switching losses and degrade the power factor. As a result, it is extremely difficult to operate such converters at a high switch frequencies (i.e. 20 KHz or higher) without implementing soft-switching technique.
A lot of research has been spent on improving the prior art rectifier and inverter circuits, the major thrust being on pulse width modulation (PWM) strategies. Though many useful soft-switching PWM strategies have been developed, none are completely satisfactory. The most advanced available soft-switching techniques are the resonant DC link, the quasi-resonant DC link, and the space-vector based zero-voltage transition. The major drawback of the resonant DC link technique is that the resonant components appear in the main power path and the resonance increases the voltage or current stresses of the switches. The quasi resonant DC link technique requires more complicated control and produces more circulating energy causing high conduction losses. The space-vector based zero-voltage transition technique can only be implemented with high speed digital signal processor and requires many auxiliary components. Additionally, all these techniques are only about zero-voltage switching. Until now, a suitable zero-current switching technique has not been developed.